Process and apparatus for the manufacture of solutions of alkali metal hydroxides



Feb. 18, 1958 R. A. CRABBE 2,824,054

PROCESS AND APPARATUS FOR THE MANUFACTURE-OF SOLUTIONS OF ALKALI METALHYDROXIDES Filed Jan. 4, 1955 United States Patent ice PROCESS ANDAPPARATUS FOR THE MANUFAC- TURE 0F SGLUTIONS 0F ALKALI METAL HY-DROXIDES Ren Al Crabbe, Boitsfort-Brussels, Belgium, assignor to Solvay& Cie., Brusseis, Belgium, a Belgian company Application January 4,1955, Serial No. 479,704 Claims priority, application Belgium January 6,1954 11 Claims. (Cl. 204--99) This invention relates to a process forthe manufacture of solutions of alkali metal hydroxides by thedecomposition of am-algams obtained by electrolysis of the correspondingsalts such as for example chlorides or sulphates of sodium or potassium.

The decomposition of the 'amalgams is carried out in apparatus whosefunctioning can be compared to that of a so called denuder cell in whichthe amalgam forming the anode is decomposed in contact with the cathodeconstituted, for example, by a set of successive graphite grids. Theapparatus for the decomposition of amalgams is often called a denuder.

It is known that the activity of denuders depends on that of thecathodes and that this activity progressively decreases with use.Immediately after having replaced a set of cathode elements it ispossible to work in the presence of solutions having elevatedconcentration of alkali metal hydroxide, but gradually it becomesnecessary to work with more dilute solutions in order to decompose theamalgam at a still satisfactory rate. Finally it becomes necessary toreplace the cathode elements.

As in practice, the tendency prevails to produce more and more highlyconcentrated solutions, it is necessary to replace the cathodes more andmore frequently. These replacements provoke disturbances in the normalworking of the denuders. Independently of the fluctuationsin the alkalimetal hydroxide concentration of the lyes, suspension of solidimpurities in the lyes is produced by the handling of the grids therein.Another disadvantage derives from the fact that in the case of totalreplacement of the set of cathode elements of a denuder it is necessaryto stop the cell which supplies the denuder with amalgam.

Attempts have been made to obviate these disadvantages by improving theconstruction of the elements constituting the cathode so as to impart tothem a prolonged activity. Up to now, it has, however, not been possibleto avoid the said disturbances. f

It is an object of the invention to provide a process which permits toproduce very pure solutions of alkali metal hydroxides at elevatedconcentration in a continuous and indefinitely prolonged manner bykeeping constant the total activity of the whole set of cathodes of adenuder.

In denuders of the horizontal type the cathode layer is. formed byelements which have the form of a grid whose graphite bars form thecathode elements proper. These bars can be in direct contact with theamalgam to be decomposed; they can likewise be kept at some distancefrom the amalgam layer, but in this case the shortcircuit of the denuderis safeguarded by contact pieces, preferably metallic ones, which areimmersed in the mercury. These graphite bars lose finally their activityof decomposition even if they have not become white or have not acquireda metallic look. This loss of activity can be explained by the factthat, in the decomposition of amalgam, not only the ions of the alkalimetal are liberated and react with water, but also, by the fact2,824,054 Patented Feb. 18, 1958 that under the action of the generatedelectric current, metallic contaminations of the amalgam decompose onthe cathode bars and modify the electric characteristics of same.

The process of the invention comprises the moving of the set of gridsfrom one end to the other end of the denuder at a rate which will be inaccordance with the rate of the activity loss of the graphite, removingone grid at said last end and placing of a fresh or regenerated gridinto the space which has become free in the denuder. In practice, thisdisplacement can be carried out in a discontinuous manner, but it ispossible to provoke a slow but continuous displacement withouttransgressing the scope of the invention.

The grids removed from the end of the denuder are then regenerated inknown manner, such as for example, by steaming, or by applyingmechanical means suitable for the cleaning of surfaces, etc.

It has been found that by applying the process according to theinvention there are obtained alkali metal hydroxides which are superiorin purity to those obtained by known processes.

The new process is applicable to amalgam of alkali metals howeverobtained. When the latter are obtained from an electrolytic cell whichis coupled with the denuder (an arrangement usually applied in theelectrolysis of aqueous solutions) the working. of the denuder has to beinterrupted when it is necessary to replace the set of worn anodes, evenif the said cell comprises a device for compensating for the wear of theanodes during working. On the contrary, it is conceivable that thecontinuous working of the denuder could be unlimited when the samedenuder receives the amalgam from two or more cells. This uninterruptedworking may also be ensured by coupling the denuder according to theinvention with an electrolytic cell or" the type described in Belgianspecification 499,224 of Nov. 7, 1950, according to which the anodes aresuccessively replaced in proportion as they wear down.

The invention also comprises a device for carrying out the process in adenuder of the horiontal type substantially consisting in providing thecell with a compartment for introducing fresh cathodes and a compartmentfor removing the least active cathodes, the compartment placed on theside of the inlet of the concentrated alkali metal amalgam, beingadvantageously provided with a guide which prevents the contact betweenthe cathode and the concentrated amalgam.

The accompanying drawing illustrates diagrammatically and by way ofexample a longitudinal section through a denuder for carrying out theprocess according to the present invention.

As shown in the drawing the denuder comprises a tank 1, an inlet 2 forthe amalgam, an outlet 3 for the mercury, an outlet 4 for the lye, and ahydraulic mercury guard 5. The tank is closed by a lid 6 provided withgas outlets 7. The compartment of decom osition of amalgam is delimitedby two weirs 3 and d which are gas-tight and immersed in the solution ofalkali metal hydroxide It The compartment for the decomposition issituated between two other compartments: the first of which 11 servesfor introducing fresh cathodes and the second one 12 which serves forremoving the cathodes which have become inactive.

The introduction of fresh cathodesis carried out while the denuder isworking, and it is therefore necessary to avoid any contact betweenthese cathodes and the amalgam at the moment of their introduction. Forthis reason, the compartment for introducing the cathodes is pro videdwith a slide 13 which keeps the fresh cathodes above the amalgam andconducts them directly into the decomposition compartment. Thecompartment for removing cathodes may similarly be provided with a slide20. When a fresh cathode 14 is introduced the whole series or train ofcathodes 15 moves forward by the range of one cathode, after the oldestone 16 has previously been removed in order to allow the translation ofthe other cathodes along the amalgam 17. The compartment for introducingnew cathodes and the compartment for withdrawing used cathodes areprovided with hinged lids 18 and 19. The apparatus comprises also aninlet (not shown) for introducing fresh water into the denuder.

The drawing is not limitative, as any design of denuders of thehorizontal type may be used if it can be fitted with an entrance andexit compartment for the cathodes, provided that the compartment at theside of the concentrated amalgam is furnished with a device avoiding thecontact between the fresh cathode and the concentrated amalgam as longas the said cathode remains outside the decomposition compartment. Thesaid entrance compartment may be situated also at the other end of theapparatus.

I claim:

i. In a process for continuously producing an alkali metal hydroxidesolution of uniform high concentration and purity by the decompositionof alkali metal amalgams in an elongated decomposition zone in thepresence of a plurality of graphite cathode elements, said elementsdecreasing in activity from a first end of said elongated decompositionzone to a second end thereof, the steps which comprise removing fromsaid second end of said zone the cathode element of least activity,introducing a fresh cathode element into said first end of said zone,and moving all of the cathode elements toward said second end of saiddecomposition zone a distance corresponding to that occupied by thecathode element removed.

2. In a process for continuously producing an alkali metal hydroxidesolution of uniform high concentration and purity by the decompositionof alkali metal amalgams in an elongated decomposition zone in thepresence of a plurality of graphite cathode elements, said elementsdecreasing in activity from a first end of said elongated decompositionzone to a second end thereof, the steps which comprise removing fromsaid second end of said zone the cathode element of least activity,introducing a fresh cathode element into said first end of said zone,and moving all of the cathode elements toward said second end of saiddecomposition zone a distance corresponding to that occupied by thecathode element removed, and repeating said steps a plurality of times.

3. In a process for continuously producing an alkali metal hydroxidesolution of uniform high concentration and purity by the decompositionof alkali metal amalgams in an elongated decomposition Zone in thepresence of a plurality of graphite cathode elements, said elementsdecreasing in activity from a first end of said elongated decompositionzone to a second end thereof, the steps which comprise removing fromsaid second end of said zone the cathode element of least activity,introducing a fresh cathode element into said first end of said zone,and moving all of the cathode elements toward said second end of saiddecomposition zone a distance corresponding to that occopied by thecathode element removed, and repeating said steps a plurality of timesat a rate related to a predetermined concentration and purity of saidalkali metal hydroxide solution.

4. A denuding cell of the horizontal type for continuously producing analkali metal hydroxide solution of high and substantially constantstrength by the decomposition of the corresponding alkali metal amalgam,said cell comprising, in combination, an elongated substantiallyhorizontal trough having a sole for supporting the amalgam as it flowsthrough the cell, means defining an inlet compartment at a first end ofsaid trough, means defining an outlet compartment at a second end ofsaid trough, and a main reaction chamber between said compartments, aplurality of graphite cathode elements disposed in laterally adjacentrelationship extending from said inlet compartment to said outletcompartment in a position for electrically-conductive contact with saidamalgam, said inlet and said outlet compartments each being of a sizesulficient to accommodate a cathode element outside of said mainreaction chamber, and said means defining said compartments permittingmovement of said cathode elements to and from said compartments and saidmain reaction chamber, at least said inlet compartment being providedwith guide means for supporting a cathode element outside of said mainreaction chamber and holding it out of all contact with said amalgamuntil it is introduced into the main reaction chamber, and said guidemeans in said inlet compartment being constructed to guide said cathodeelement automatically from a position substantially above the amalgam tothe position occupied by said cathode elements in the main reactionchamber.

5. A denuding cell of the horizontal type for continuously producing analkali metal hydroxide solution of high and substantially constantstrength by the decomposition of the corresponding alkali metal amalgam,said cell comprising, in combination, an elongated substantiallyhorizontal trough having a sole for supporting the amalgam as it flowsthrough the cell, means defining an inlet compartment at a first end ofsaid trough, means defining an outlet compartment at a second end ofsaid trough, and a main reaction chamber between said compartments, aplurality of graphite cathode elements disposed in laterally adjacentrelationship extending from said inlet compartment to said outletcompartment, said cathode elements being positioned to be in directcontact with said amalgam, said inlet and said outlet compartments eachbeing of a size sufficient to accommodate a cathode element outside ofsaid main reaction chamber, and said means defining said compartmentspermitting movement of said cathode elements to and from saidcompartments and said main reaction chamber, at least said inletcompartment being provided with guide means for supporting a cathodeelement outside of said main reaction chamber and holding it out of allcontact with said amalgam until it is introby said cathode elements inthe main reaction chamber.

-6. A denuding cell of the horizontal type for continuously producing analkali metal hydroxide solution of high and substantially constantstrength by the decomposition of the corresponding alkali metal amalgam,said cell comprising, in combination, an elongated substantiallyhorizontal trough having a sole for supporting the amalgam as it flowsthrough the cell, means defining an inlet compartment at a first end ofsaid trough, means defining an outlet compartment at a second end ofsaid trough, and a main reaction chamber between said compartments, aplurality of graphite cathode elements disposed in laterally adjacentrelationship extending from said inlet compartment to said outletcompartment, and electrically-conducting contact pieces for separatingsaid cathode elements from the amalgam but maintaining them inelectrically-conducting contact therewith, said inlet and said outletcompartments each being of a size sufiicient to accommodate a cathodeelement outside of said main reaction chamber, and said means definingsaid compartments permitting movement of said cathode elements to andfrom said compartments and said main reaction chamber, at least saidinlet compartment being provided with guide means for supporting acathode element outside of said main reaction chamber and holding it outof all contact with said amalgam until it is introduced into the mainreaction chamber, and said guide means in said inlet compartment beingconstructed to guide said cathode element automatically from a positionsubstantially above the amalgam to the position occupied by said cathodeelements in the main reaction chamber.

ously producing an alkali metal hydroxide solution of high andsubstantially constant strength by the decomposition of thecorresponding alkali metal amalgam, said cell comprising, incombination, an elongated substantially horizontal trough having a solefor supporting the amalgam asfit flows through the cell, means definingan inlet compartment at a first end of said trough, means defining anoutlet compartment at a second end of said trough, and a main reactionchamber between said compartments, a plurality of graphite cathodeelements disposed in laterally adjacent relationship extending from saidinlet compartment to said outlet compartment in a position forelectrically-conductive contact with said amalgam, said inlet and saidoutlet compartments each being of a size sufficient to accommodate acathode element outside of said main reaction chamber, and said meansdefining said compartments permitting movement of said cathode elementsto and from said compartments and said main reaction chamber, said mainreaction chamber having a cover and said compartments having independentcovers whereby the end compartments may be opened without opening themain reaction chamber, at least said inlet compartment being providedwith guide means for supporting a cathode element outside of said mainreaction chamber and holding it out of all contact with said amalgamuntil it is introduced into the main reaction chamber, and said guidemeans in said inlet compartment being constructed to guide said cathodeelement automatically from a position substantially above the amalgam tothe position occupied by said cathode elements in the main reactionchamber.

8. A denuding cell of the horizontal type for continuously producing analkali metal hydroxide solution of high and substantially constantstrength by the decomposition of the corresponding alkali metal amalgam,said cell comprising, in combination, an elongated substantiallyhorizontal trough having a sole for supporting the amalgam as it flowsthrough the cell, means defining an inlet compartment at a first end ofsaid trough, means defining an outlet compartment at a second end ofsaid trough, and a main reaction chamber between said compartments, aplurality of graphite cathode elements disposed in laterally adjacentrelationship extending from said inlet compartment to said outletcompartment in a position for electricallyconductive contact with saidamalgam, said inlet and said outlet compartments each being of a sizesufiicient to accommodate a cathode element outside of said mainreaction chamber, and said means defining said compartments permittingmovement of said cathode elements to and from said compartments and saidmain reaction chamber, said inlet compartment and said outletcompartment being provided with guide means for supporting a cathodeelement outside of said main reaction chamber and holding it out ofcontact with said amalgam, said guide means in said inlet compartmentbeing constructed to guide Said cathode element automatically from aposition substantially above the amalgam to the position occupied bysaid cathode elements in the main reaction chamber.

9. A denuding cell of the horizontal type for continuously producing analkali metal hydroxide solution of high and substantially constantstrength by the decomposition of the corresponding alkali metal amalgam,said cell comprising, in combination, an elongated substantiallyhorizontal trough adapted to contain the solution at a predeterminedlevel and having a sole for supporting the amalgam as it flows throughthe cell and a top, means defining an inlet compartment at a first endof said trough, means defining an outlet compartment at a second end ofsaid trough, and a main reaction chamber between said compartments, aplurality of graphite cathode elements disposed in laterally adjacentrelationship extending from said inlet compartment to said outletcompartment in a position for electrically-conductive contact with saidamalgam, said inlet and said outlet compartments each being of a sizesutficient to accommodate a cathode element outside of said mainreaction chamber, and said means defining said compartments permittingmovement of said. cathode elements to and from said compartments andsaid main reaction cham ber, said means defining the inlet compartmentbeing a weir extending from the top of the trough to a point below saidlevel whereby to be immersed in the solution, at least said inletcompartment being provided with guide means for supporting a cathodeelement outside of said main reaction chamber and holding it out of allcontact with said amalgam until it is introduced into the main reactionchamber, and said guide means in said inlet compartment beingconstructed to guide said cathode element automatically from a positionsubstantially above the amalgam to the position occupied by said cathodeelements in the main reaction chamber.

10. A denuding cell of the horizontal type for continuously producing analkali metal hydroxide solution of high and substantially constantstrength by the decomposition of the corresponding alkali metal amalgam,said cell comprising, in combination, an elongated sub stantiallyhorizontal trough having a sole for supporting the amalgam as it flowsthrough the cell, means defining an inlet compartment at a first end ofsaid trough, means defining an outlet compartment at a second end ofsaid trough, and a main reaction chamber between said compartments, aplurality of graphite cathode elements disposed in laterally adjacentrelationship extending from said inlet compartment to said outletcompartment in a position for electrically-conductive contact with saidamalgam, said inlet and said outlet compartments each being of a sizesufiicient to accommodate a cathode element outside of said mainreaction chamber, and said means defining said compartments permittingmovement of said cathode elements to and from said compartments and saidmain reaction chamber, said inlet compartment being provided with guidemeans for supporting a cathode element outside of said main reactionchamber and holding it out of all contact with said amalgam until it isintroduced into the main reaction chamber, said guide means beingconstructed to guide said cathode element from a position substantiallyabove the amalgam to the position occupied by said cathode elements inthe main reaction chamber, and said outlet compartment containing guidemeans to facilitate withdrawal of the cathode of least activity.

11. A denuding cell of the horizontal type for continuously producing analkali metal hydroxide solution of high and substantially constantstrength by the decomposition of the corresponding alkali metal amalgam,said cell comprising, in combination, an elongated substantiallyhorizontal trough adapted to contain the solution at a predeterminedlevel and having a sole for supporting the amalgam as it flows throughthe cell and a top, means defining an inlet compartment at a first endof said trough, means defining an outlet compartment at a second end ofsaid trough, and a main reaction chamber between said compartments, aplurality of graphite cathode elements disposed in laterally adjacentrelationship extending from said inlet compartment to said outletcompartment in a position for electrically-conductive contact with saidamalgam, said inlet and said outlet compartments each being of a sizesufficient to accommodate a cathode element outside of said mainreaction chamber, and said means defining said compartments permittingmovement of said cathode elements to and from said compartments and saidmain reaction chamber, said means defining the outlet compartment beinga weir extending from the top of the trough to a point below said levelwhereby to be immersed in the solution, at least said inlet compartmentbeing provided with guide means for supporting a cathode element outsideof said main reaction chamber and holding it out of all contact with 7said amalgam until it is introduced into the main reaction chamber, andsaid guide means in said inlet compartment being constructed to guidesaid cathode element automatically from a position substantially abovethe amalgam to the position occupied by said cathode elements in the 5main reaction chamber.

8 References Cited in the file of this patent UNITED STATES PATENTS2,551,248 Deprez et a1. May 1, 1951 FOREIGN PATENTS 235,756 SwitzerlandApr. 16, 1945

1. IN A PROCESS FOR CONTINUOUSLY PRODUCING AN ALKALI METAL HYDROXIDESOLUTION OF UNIFORM HIGH CONCENTRATION AND PURITY BY THE DECOMPOSITIONOF ALKALI METAL AMALGANS IN AN ELONGATED DECOMPOSITION ZONE IN THEPRESENCE OF A PLURALITY OF GRAPHITE CATHODE ELEMENTS, SAID ELEMENTSDECREASING IN ACTIBITY FROM A FIRST END OF SAID ELONGATED DECOMPOSITIONZONE TO A SECOND END THEREOF, THE STEPS WHICH COMPRISE REMOVING FROMSAID SECOND END OF SAID ZONE THE CATHODE ELEMENT OF LEAST ACTIVITYINTRODUCING A FRESH CATHODE ELEMENT INTO SAID FIRST END OF SAID ZONE,AND MOVING ALL OF THE CATHODE ELEMENTS TOWARD SAID SECOND END OF SAIDDECOMPOSITION ZONE A DISTANCE CORRESPONDING TO THAT OCCUPIED BY THECATHODE ELEMENT REMOVED.